Johnson thermo-electrochemical converter

ABSTRACT

A electrochemical direct heat to electricity converter having a low temperature membrane electrode assembly array and a high temperature membrane electrode assembly array is provided. Additional cells are provided in the low temperature membrane electrode assembly array, which causes an additional amount of the working fluid, namely hydrogen, to be pumped to the high pressure side of the converter. The additional pumped hydrogen compensates for the molecular hydrogen diffusion that occurs through the membranes of the membrane electrode assembly arrays. The MEA cells may be actuated independently by a controller to compensate for hydrogen diffusion

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.16/333,801, filed on Mar. 15, 2019 and allowed on Nov. 23, 2021, whichis a Section 371 of International Application No. PCT/US2017/052191,filed Sep. 19, 2017, which was published in the English language on Mar.22, 2018, under International Publication No. WO 2018/053474 A1, whichclaims priority to U.S. Provisional Application No. 62/396,463 filed onSep. 19, 2016, the disclosures of each of which are incorporated hereinby reference in their entireties.

BACKGROUND OF THE INVENTION

The conversion of heat energy or chemical energy to electrical energy,or visa-versa, may be accomplished in a variety of ways. For example,known electrochemical cells or batteries rely on chemical reactionswherein ions and electrons of a reactant being oxidized are transferredto the reactant being reduced via separate paths. Specifically, theelectrons are transferred electrically via wiring through an externalload where they perform work and the ions are conducted through anelectrolyte separator.

However, battery type electrochemical cells can produce only a limitedamount of energy because the confines of the battery casing limit theamount of available reactants that may be contained therein. Althoughsuch cells can be designed to be recharged by applying a reversepolarity current/voltage across the electrodes, such recharging requiresa separate electrical source. Also, during the recharging process, thecell is typically not usable.

Fuel cells have been developed in an effort to overcome problemsassociated with battery type electrochemical cells. In conventional fuelcells, the chemical reactants are continuously supplied to and removedfrom the electrochemical cell. In a manner similar to batteries, fuelcells operate by conducting an ionized species through a selectiveelectrolyte which generally blocks passage of electrons and non-ionizedspecies.

The most common type of fuel cell is a hydrogen-oxygen fuel cell whichpasses hydrogen through one of the electrodes and oxygen through theother electrode. The hydrogen ions are conducted through the electrolyteseparator to the oxygen side of the cell under the chemical reactionpotential of the hydrogen and oxygen. Porous electrodes on either sideof the electrolyte separator are used to couple the electrons involvedin the chemical reaction to an external load via an external circuit.The electrons and hydrogen ions reconstitute hydrogen and complete thereaction, while the oxygen on the oxygen side of the cell results in theproduction of water which is expelled from the system. A continuouselectrical current is maintained by a continuous supply of hydrogen andoxygen to the cell.

Mechanical heat engines have also been designed and used to produceelectrical power. Such mechanical heat engines operate on thermodynamiccycles wherein shaft work is performed using a piston or turbine tocompress a working fluid. The compression process is performed at a lowtemperature and, after compression, the working fluid is raised to ahigher temperature. At the high temperature, the working fluid isallowed to expand against a load, such as a piston or turbine, therebyproducing shaft work. A key to the operation of all engines employing aworking fluid is that less work is required to compress the workingfluid at low temperatures than that produced by expanding it at hightemperatures. This is the case for all thermodynamic engines employing aworking fluid.

For example, steam engines operate on the Rankine thermodynamic cycle,wherein water is pumped to a high pressure, and then heated to steam andexpanded through a piston or turbine to perform work. Internalcombustion engines operate on the Otto cycle, wherein low-temperatureambient air is compressed by a piston and then heated to very hightemperatures via fuel combustion inside the cylinder. As the cyclecontinues, the expansion of the heated air against the piston producesmore work than that consumed during the lower temperature compressionprocess.

The Stirling engine has been developed to operate on the Stirling cyclein an effort to provide an engine that has high efficiency and offersgreater versatility in the selection of the heat source. The idealStirling thermodynamic cycle is of equivalent efficiency to the idealCarnot cycle, which defines the theoretical maximum efficiency of anengine operating on heat input at high temperatures and heat rejectionat low temperatures. However, as with all mechanical engines, theStirling engine suffers from reliability problems and efficiency lossesassociated with its mechanical moving parts.

In an effort to avoid the problems inherent with mechanical heatengines, Alkali Metal Thermo-Electrochemical Conversion (AMTEC) cellshave been designed as a thermo-electrochemical heat engine. AMTEC heatengines utilize pressure to generate a voltage potential and electricalcurrent by forcing an ionizable working fluid, such as sodium, throughan electrochemical cell at high temperatures. The electrodes couple theelectrical current to an external load. Electrical work is performed asthe pressure differential across the electrolyte separator forces moltensodium atoms through the electrolyte. The sodium is ionized uponentering the electrolyte, thereby releasing electrons to the externalcircuit. On the other side of the electrolyte, the sodium ions recombinewith the electrons to reconstitute sodium upon leaving the electrolyte,in much the same way as the process that occurs in battery and fuel celltype electrochemical cells. The reconstituted sodium, which is at a lowpressure and a high temperature, leaves the electrochemical cell as anexpanded gas. The gas is then cooled and condensed back to a liquidstate. The resulting low-temperature liquid is then re-pressurized.Operation of an AMTEC engine approximates the Rankine thermodynamiccycle.

Numerous publications are available on AMTEC technology. See, forexample, Conceptual design of AMTEC demonstrative system for 100 t/dgarbage disposal power generating facility, Qiuya Ni et al. (ChineseAcademy of Sciences, Inst. of Electrical Engineering, Beijing, China).Another representative publication is Intersociety Energy ConversionEngineering Conference and Exhibit (IECEC), 35th, Las Vegas, Nev. (Jul.24-28, 2000), Collection of Technical Papers. Vol. 2 (A00-37701 10-44).Also see American Institute of Aeronautics and Astronautics, 190, p.1295-1299. REPORT NUMBER(S)—AIAA Paper 2000-3032.

AMTEC heat engines suffer from reliability issues due to the highlycorrosive nature of the alkali metal working fluid. AMTEC engines alsohave very limited utility. Specifically, AMTEC engines can only beoperated at very high temperatures because ionic conductive solidelectrolytes achieve practical conductivity levels only at hightemperatures. Indeed, even the low-temperature pressurization processmust occur at a relatively high temperature, because the alkali metalworking fluid must remain above its melt temperature at all times as itmoves through the cycle. Mechanical pumps and even magneto-hydrodynamicpumps have been used to pressurize the low-temperature working fluid.

In an effort to overcome the above-described drawbacks of conventionalmechanical and thermo-electrochemical heat engines, the JohnsonThermo-Electrochemical Converter (JTEC) system was developed, asdisclosed in U.S. Pat. No. 7,160,639 filed Apr. 28, 2003, InternationalPatent Application No. PCT/US2015/044435 filed Aug. 10, 2015, andInternational Patent Application No. PCT/US2016/21508 filed Mar. 9,2016, the entire contents of all three documents being incorporatedherein by reference.

The present invention provides an improvement over a typical JTECsystem. More particularly, the present invention provides a solid stateheat engine that compensates for pressure loss due to the diffusion ofmolecular hydrogen.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a electrochemicaldirect heat to electricity converter having a low temperature membraneelectrode assembly array and a high temperature membrane electrodeassembly array. Additional cells are provided in the low temperaturemembrane electrode assembly array as compared to the high temperaturemembrane electrode assembly array, which causes additional workingfluid, namely hydrogen, to be pumped to the high pressure side of theconverter. The additional pumped hydrogen compensates for the molecularhydrogen diffusion that occurs through the membranes of the membraneelectrode assembly arrays. In one embodiment, the MEA cells may beactuated independently by a controller to compensate for hydrogendiffusion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of preferred embodiments of thepresent invention will be better understood when read in conjunctionwith the appended drawing. For the purposes of illustrating theinvention, there is shown in the drawing an embodiment which ispresently preferred. It is understood, however, that the invention isnot limited to the precise arrangements and instrumentalities shown. Inthe drawings:

FIG. 1 is a diagram of a Johnson Thermo-Electrochemical Converterincluding two membrane electrode assemblies connected back to back by arecuperative heat exchanger;

FIG. 2 is a plot of Nernst voltage versus temperature for severalpressure ratios;

FIG. 3 is a diagram of the Ericsson thermodynamic cycle;

FIG. 4 is a schematic of the operating configuration of a conventionalJohnson Thermo-Electrochemical Converter;

FIG. 5 is an operation start up diagram of the engine, in accordancewith an embodiment of the present invention, with an external powersource and a control switch showing hydrogen being pumped to the highpressure side of the engine;

FIG. 6 is a schematic of the operating configuration of the engine withadditional MEA cells on the low temperature side to compensate for lossof pressure differential due to hydrogen diffusion across the membranes,in accordance with an embodiment of the present invention;

FIG. 7 is a schematic of the operating configuration of the engine withMEA cell arrays and a controller to compensate for loss of pressuredifferential due to hydrogen diffusion across the membranes, inaccordance with an embodiment of the present invention; and

FIG. 8 is a diagram representing the operating configuration of theengine at a low grade temperature of 150° C. and including additionalMEA cells on the low pressure side to compensate for loss of pressuredifferential due to hydrogen diffusion across the membranes, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “proximal,” “distal,” “upward,”“downward,” “bottom” and “top” designate directions in the drawings towhich reference is made. The words “inwardly” and “outwardly” refer todirections toward and away from, respectively, a geometric center of thedevice, and designated parts thereof, in accordance with the presentinvention. Unless specifically set forth herein, the terms “a,” “an” and“the” are not limited to one element, but instead should be read asmeaning “at least one.” The terminology includes the words noted above,derivatives thereof and words of similar import.

It will also be understood that terms such as “first,” “second,” and thelike are provided only for purposes of clarity. The elements orcomponents identified by these terms, and the operations thereof, mayeasily be switched. Also, the phrases “plurality of MEAs”, “MEA cellarray” and “MEA cell stack” may be used interchangeably herein.

Referring to FIG. 1, there is shown a typical JTEC system (electricalconnections not shown). JTEC is a heat engine that includes a firstelectrochemical cell 100 operating at a relatively low temperature, asecond electrochemical cell 110 operating at a relatively hightemperature, a conduit system 112 including a heat exchanger 114 thatcouples the two cells together, and a supply of ionizable gas (such ashydrogen, oxygen or sodium) as a working fluid contained within theconduit system. Preferably, the working fluid is hydrogen. Eachelectrochemical cell includes a Membrane Electrode Assembly (MEA).

More particularly, the JTEC heat engine includes a first MEA 118 coupledto a high temperature heat source Q_(H) (i.e., a high temperature MEA),a second MEA 116 coupled to a low temperature heat sink Q_(L) (i.e., alow temperature MEA), and a recuperative heat exchanger 114 connectingthe two MEAs 116, 118. Each MEA 116, 118 includes a non-porous membrane120 capable of conducting ions of the working fluid and porouselectrodes 122 positioned on opposite sides of the non-porous membrane120 that are capable of conducting electrons.

Preferably, on both the high temperature and low temperature sides,overlapping layers of alternating electrodes 122 and membranes 120arranged in a stacked configuration. That is, each membrane 120 issandwiched between a pair of electrodes 122, such that the electrodes122 are stacked in alternating sequence with the membranes 120, thusforming MEA stacks through which a working fluid, preferably hydrogen,can pass by undergoing an electrochemical oxidation/reduction process.

The membranes 120 are preferably ion conductive membranes or protonconductive membranes having a thickness on the order of approximately0.1 μm to 500 μm, and more preferably between approximately 1 μm and 500μm. More particularly, the membranes 120 are preferably made from aproton conductive material, and more preferably a polymer protonconductive material or a ceramic proton conductive material. In oneembodiment, the membranes 120 are preferably formed of a materialcomprising a compound represented by the general formula Na_(x) Al_(y)Ti³⁺ _(x-y) Ti⁴⁺ _(8-x) O₁₆, as disclosed in U.S. Pat. No. 4,927,793 ofHori et al., which is incorporated herein by reference, since thismaterial exhibits high proton conductivity over a broad temperaturerange. However, it will be understood by those skilled in the art thatany material, and preferably any polymer or ceramic material, whichdemonstrates a similar proton conductivity over a broad temperaturerange may be used to form the membranes 120. For example, in analternate embodiment, the membranes 120 are formed of hydronium beta″alumina.

The electrodes 122 are preferably thin electrodes having a thickness onthe order of approximately 10 μm to 1 cm, and more preferablyapproximately 50 μm to 1,000 μm. The use of different materials for thevarious components (i.e., the electrodes 122 and the membranes 120)could result in very high thermal stresses due to differences in thethermal expansion coefficients between the materials. Accordingly, theelectrodes 122 are preferably comprised or formed of the same materialas the membranes 120. However, the electrodes 122 are preferably porousstructures, while the membranes 120 are preferably non-porousstructures. Also, it will be understood that the electrodes 122 and themembranes 120 may be formed of different materials having similarthermal expansion coefficients.

In one embodiment, the porous electrodes 122 may be doped or infusedwith additional material(s) to provide electronic conductivity andcatalytic material, in order to promote oxidation and reduction of theworking fluid.

On both the low temperature and high temperature sides, individual MEAs116, 118 are preferably connected in series to form MEA stacks orarrays.

During operation of the JTEC, the working fluid passes through each MEAstack by releasing an electron to the electrode 122 on the enteringside, such that the ion can be conducted through the membrane 120 to theopposite electrode 122. The working fluid is reconstituted within theopposite electrode 122 as it re-supplies electrons to working fluid ionsas they exit the membrane 120. The low temperature MEA stack operates ata lower voltage than the high temperature MEA stack. The low temperatureMEA stack compresses the working fluid at low voltage and the hightemperature MEA stack expands hydrogen at high voltage. The differencein voltage between the two MEA stacks is applied across the externalload. The hydrogen circulates continuously inside the JTEC heat engineand is never consumed. The current flow through the two MEA stacks andthe external load is the same.

Specifically, in the JTEC heat engine, a hydrogen pressure differentialis applied across each MEA stack with a load attached, thereby producinga voltage and current as hydrogen passes from high pressure to lowpressure. The electron current is directed to the external load aselectrons are stripped from the protons as they pass through themembrane 120, which is a proton conductive membrane (PCM). The JTECsystem utilizes the electrochemical potential of hydrogen pressureapplied across the PCM 120. More particularly, on the high pressure sideof the first MEA stack (i.e., the stack of first MEAs 116) and the lowpressure side of the second MEA stack (i.e, the stack of second MEAs118), hydrogen gas is oxidized resulting in the creation of protons andelectrons. The pressure differential at the high temperature end forcesthe protons through the membrane 120 causing the electrodes 122 toconduct electrons through an external load, while the imposition of anexternal voltage forces protons through the membrane at the lowtemperature end. On the high pressure side of the first MEA stack andthe low pressure side of the second MEA stack, the protons are reducedwith the electrons to reform hydrogen gas.

Unlike conventional fuel cells, in which the hydrogen exiting the MEAstack would encounter oxygen and react with it producing water, there isno oxygen or water in the JTEC system. This process can also operate inreverse. Specifically, if current is passed through the first MEA stack,a low-pressure gas can be “pumped” to a higher pressure. The reverseprocess is rather similar to that of using a MEA stack to electrolyzewater, wherein water molecules are split and protons are conductedthrough the PCM, leaving oxygen behind on the water side. Hydrogen isoften supplied at a high pressure to a pure hydrogen reservoir via thisprocess.

In the JTEC, using hydrogen as the ionizable gas (i.e., the workingfluid), the electrical potential due to a hydrogen pressure differentialacross the PCM 120 is proportional to the natural logarithm of thepressure ratio, and can be calculated using the Nernst equation:

$\begin{matrix}{{V_{OC} = {\frac{RT}{2F}{\ln\left( {P_{H}/P_{L}} \right)}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where V_(OC) is open circuit voltage, R is the universal gas constant, Tis the cell temperature, F is Faraday's constant, P_(H) is the pressureon the high pressure side, P_(L) is the pressure on the low pressureside, and P_(H)/P_(L) is the pressure ratio. E.g., Fuel Cell Handbook,J. H. Hirschenhofer et al., 4^(th) Edition, p. 2-5 (1999).

The voltage generated by the first MEA stack is thus given by the Nernstequation. The voltage is linear with respect to temperature and is alogarithmic function of the pressure ratio. FIG. 2 is a plot of theNernst equation for hydrogen and shows the voltage vs. temperaturerelationship for several pressure ratios. For example, referring to FIG.2, at a pressure ratio of 10,000, when the temperature is relativelyhigh, the voltage is similarly relatively high and when the temperatureis relatively low, the voltage is similarly relatively low.

The working fluid in the JTEC is compressed in the low temperatureelectrochemical cell 100 by supplying current at a voltage that issufficient to overcome the Nernst potential of the low temperature cell100, thereby driving hydrogen from the low pressure side of the membrane120 to the high pressure side. On the other hand, the working fluid isexpanded in the high temperature electrochemical cell 110 as current(power) is extracted under the Nernst potential of the high temperaturecell 110. Electrical current flow is generated as hydrogen expands fromthe high pressure side of the membrane 120 to the low pressure side. Asin any thermodynamic engine employing a working fluid and consistentwith the nature of compressible gas, in the JTEC, a greater amount ofwork (electricity) is extracted during high temperature expansion thanthe work (electricity) input required for the low temperaturecompression. The difference in heat energy input to the engine tomaintain constant temperature during high temperature expansion versusthe heat energy removed to maintain constant temperature during lowtemperature compression is provided as the difference in electricalenergy output by the high temperature expansion process versus thatconsumed by the low temperature compression process.

Consistent with the Nernst equation, the high temperature cell 110 willhave a higher voltage than the low temperature cell. Since the current(I) is the same through both cells 100, 110, the voltage differentialmeans that the power generated through the expansion of hydrogen in thehigh temperature cell 110 is higher than that of the low temperaturecell 100. The power output by the high temperature cell (V_(HT)*I) issufficient to drive the compression process in the low temperature cell100 (V_(LT)*I) as well as supply net power output to an external load((V_(HT)*I)−(V_(LT)*I)). This voltage differential provides the basisfor the JTEC engine.

The JTEC is an all solid-state engine that operates on the moreefficient Ericsson cycle, which is equivalent to the Carnot cycle.Referring to FIG. 3, there is shown the ideal temperature entropydiagram for the Ericsson engine cycle of the JTEC. Reference numerals“1” through “4” in FIGS. 1 and 3 represent different thermodynamicstates. The thermodynamic states 1 through 4 are identical at therespective identified points in FIGS. 1 and 3. As shown in FIG. 1,beginning at the low-temperature, low-pressure state 1, electricalenergy Win is supplied to the low-temperature (first) MEA stack in orderto pump hydrogen from the low-temperature, low-pressure state 1 to thelow-temperature, high-pressure state 2. The temperature of the hydrogenis maintained nearly constant by removing heat Q_(L) from the PCM 120during the compression process. The membrane 120 is relatively thin(i.e., less than 10 μm thick), and thus will not support a significanttemperature gradient, so the near isothermal assumption for the processis valid, provided adequate heat is transferred from the membrane 120through its substrate.

From state 2, the hydrogen passes through the recuperative, counterflowheat exchanger 114 and is heated under approximately constant pressureto the high-temperature state 3. The heat needed to elevate thetemperature of the hydrogen from state 2 to 3 is transferred fromhydrogen flowing in the opposite direction through the heat exchanger114. At the high-temperature, high-pressure state 3, electrical power isgenerated as hydrogen expands across the second MEA stack from thehigh-pressure, high-temperature state 3 to the low-pressure,high-temperature state 4. Heat Q_(H) is supplied to the thin filmmembrane 120 to maintain a near constant temperature as the hydrogenexpands from high-pressure state 3 to low-pressure state 4. From state 4to state 1, the hydrogen flows through the recuperative heat exchanger114, wherein its temperature is lowered by heat transfer to hydrogenpassing from state 2 to 3. The hydrogen is pumped by the low-temperatureMEA stack from state 1 back to high-pressure state 2 as the cyclecontinues.

However, PCMs are not perfect barriers to the working fluid, namelyhydrogen, and have known diffusion rates, which means that the hydrogenpressure differential, upon which operation of the JTEC engine dependsas described above, will neutralize itself by molecular diffusionthrough the PCMs. More particularly, FIG. 4 depicts this hydrogendiffusion in the prior art operation of the JTEC with a high temperatureMEA stack 204 coupled to a heat source Q_(H) and a low temperature MEAstack 203 coupled to a heat sink Q_(L). The two MEA stacks 203 and 204have the same number of cells (i.e., MEAs) and the cells are connectedin series, thus ensuring that the net current flow in each MEA stack203, 204 is the same. The result is that hydrogen circulation in eachMEA stack 203, 204 is the same and is therefore continuous. Hydrogencirculates within the converter as indicated by arrows 205 between thetwo MEA stacks 203, 204, as it is conducted back and forth between thelow pressure conduit 201 and the high pressure conduit 202. Molecularhydrogen diffusion through the membranes is represented by arrows 206.Nominally, however, over time, the pressure differential will decaybecause of the natural diffusion under the applied pressure.

Also, operation of the conventional JTEC may be initiated after a periodof dormancy during which the pressure differential may have decayed dueto molecular hydrogen diffusion. The present improvement maintains theoperating pressure differential across the membrane to compensate forthe diffusion loss or decay.

Referring to FIG. 5, there is shown the start-up operation of animproved JTEC system including a low temperature array or stack 303 of afirst plurality of MEA cells electrically connected in series and a hightemperature array or stack 304 of a second plurality of MEA cellselectrically connected in series. In this example, the electrodes of theMEA cells share a common membrane. FIG. 5 illustrates the simple processof initially applying power to one of the MEA stacks to drive hydrogen Hfrom low pressure side 301 to high pressure side 302 in order toequalize the pressure across the membranes. Once the pressuredifferential is established, the converter can be switched to its normaloperating configuration with the MEA cells connected in series with theexternal load. Power can then be generated by applying heat to the hightemperature MEA stack 304 and removing heat from low temperature MEAstack 303.

More particularly, referring to FIG. 6, the improved JTEC includesadditional MEA cells 307 in the low temperature MEA stack 303. Thus, thelow temperature MEA stack 303 includes more MEA cells than the hightemperature MEA stack 304. The supplemental MEA cells 307 are connectedin series with the rest of the MEA cells in the low temperature MEAstack 303, such that the net voltage of the low temperature MEA stack303 is slightly higher than in a conventional JTEC, but is still lessthan the net voltage of the high temperature MEA stack 304. The seriesconnection of the MEA cells in the low temperature MEA stack 303 ensuresthat the current flow through each cell in the stack 303 remains thesame. Hydrogen circulates within the converter as indicated by arrows305 between the two MEA stacks 303, 304, as it is conducted back andforth between the low pressure conduit 301 and the high pressure conduit302. However, the additional cells 307 in the low temperature MEA stack303 cause additional hydrogen 308 to be pumped from the low pressureside 301 to the high pressure side 302 of the converter. Pumping of theadditional hydrogen through the converter may be effected by the MEAstacks or arrays 303, 304 themselves, or by a controller (shown in FIG.7). Operation under this condition means that hydrogen diffusion 306 iscompensated for by the additional pumped hydrogen 308.

FIG. 7 is a diagram showing a controller for monitoring the pressures onthe high and low pressure sides of the JTEC and the temperatures at thehigh and low temperature ends of the JTEC. A controller 71 uses thepressure measurements to determine the extent of pressure differentialdiffusion loss. The controller 71 is coupled to the membrane electrodeassembly cells within the JTEC and actuates the cells to pump additionalhydrogen to the high pressure side when the pressure differential dropsbelow a predetermined value. The controller 71 may optionally monitorcell voltage and temperature and calculate the pressure differentialusing the Nernst equation.

FIG. 8 is a diagram representing the operating configuration of theengine of the present invention at a low grade temperature of 150° C.The results of FIG. 8 demonstrate the additional pumped hydrogen 308raises the pressure differential until equilibrium (i.e., equalization)is achieved, such that the pressure differential is high enough to forcehydrogen diffusion 306 to a level that is equal to the pumped flow ofhydrogen 308. In this example, the high temperature MEA stack 304 ismade up of 250 cells, whereas the low temperature MEA stack is made upof 260 cells. Because of the series connection of the cells, as the loadincreases its current draw, the imbalanced rate at which hydrogen ispumped to the high pressure side 302 also increases. The increased pumprate of the hydrogen 308 drives the pressure differential higher, untilthe back diffusion rate 306 of the hydrogen against the pressuredifferential becomes high enough to achieve equilibrium with the pumprate 308. The diagram of FIG. 8 shows the increase in pressure with anincrease in output current.

Similar to conventional fuel cells, the JTEC operates under maximumefficiency levels when operating at lowest power levels. Referring toFIG. 8, the JTEC achieves a peak power density of 700 mWh/cm³ at a highside pressure of 319 psi and 35% of Carnot. At 80% of Carnot, theoperating pressure is 72 psi and the power density is 270 mW/cm³. Also,the open circuit voltage increases as the current draw increases due tothe increase in the pressure ratio, as described above. However, theoutput voltage decreases with increasing current draw, because thehigher current causes increased voltage drop across the internalresistance of the MEA cells.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

I claim:
 1. A thermo-electrochemical converter comprising a firstmembrane electrode assembly array and a second membrane electrodeassembly array, each of the first and second membrane electrode assembly(MEA) arrays comprising: a plurality of porous electrodes; a workingfluid; a plurality of ion or proton conductive membranes, the porouselectrodes being arranged in an alternating sequence with the membranes;a plurality of conduits containing the working fluid, at least one ofthe conduits being a high pressure conduit containing the working fluidat a first pressure and at least another one of the conduits being a lowpressure conduit containing the working fluid at a second pressure whichis lower than the first pressure, wherein the number of porouselectrodes and membranes of the first MEA array is greater than thenumber of porous electrodes and membranes of the second MEA array,wherein a first porous electrode of any sequential pair of the porouselectrodes in each MEA array is coupled to the low pressure conduit forlow pressure working fluid flow therethrough and a second porouselectrode of the sequential pair of the porous electrodes is coupled tothe high pressure conduit for high pressure working fluid flowtherethrough, such that each of the membranes is subjected to a pressuredifferential between the pair of porous electrodes, and wherein thegreater number of porous electrodes and membranes of the first MEA arraycauses an additional amount of the working fluid to be pumped from a lowpressure side to a high pressure side of the converter, in order tocompensate for a loss of the pressure differential resulting frommolecular diffusion of the working fluid through the membranes.
 2. Thethermo-electrochemical converter according to claim 1, wherein theporous electrodes of the first MEA array are electrically connected inseries with each other so as to produce a higher MEA voltage.
 3. Thethermo-electrochemical converter according to claim 1, wherein theworking fluid is selected from a group consisting of oxygen, hydrogenand sodium.
 4. The thermo-electrochemical converter according to claim3, wherein the working fluid is hydrogen.
 5. The thermo-electrochemicalconverter according to claim 1, further comprising heat exchanger meansfor transferring heat from the high pressure conduit to the low pressureconduit.
 6. The thermo-electrochemical converter according to claim 1,further comprising a controller which actuates the MEA arrays to pumpthe additional hydrogen to the high pressure side.